1. Field of the Invention
The present invention relates to improved siliceous glass-forming systems, the compositions therefor being high in silicon content, and, more especially, to such compositions comprising the lead or alkaline earth metal silicates. The invention also relates to the preparation of such vitrescible siliceous/silicate compositions and to the manufacture of glass therefrom.
2. Description of the Prior Art
It is well known to this art that many different materials are useful for the manufacture of glass. Silicon, boron aluminum and phosphorus, in oxide form, are the conventional elements of the three-dimensional, random glass network, or lattice. The respective amounts of such components in the glass are ofttimes limited by the difficulties encountered in obtaining fusion, fining and homogenization under acceptable conditions.
Fluxes, to lower melting and working temperatures, are also common, e.g., the oxide forms of lithium, sodium and potassium, etc. However, such additives impart unsatisfactory properties vis-a-vis the intended applications of the glasses shaped from compositions comprising the same. It too is necessary to add other oxides to improve certain properties: for example, chemical resistance is enhanced by calcium oxide, the index of refraction by barium or lead, while zinc oxide adds brilliance.
Also, various other oxides are added to the glass formers for a wide variety of purposes. The oxides of arsenic, antimony, cerium and other rare earths, for example, facilitate the fining operation. Salts, such as nitrates and sulfates, are introduced for oxidation. The addition of heavy metals enables coloring. All of the components of the glass-forming system are typically introduced in a variety of forms. Silicon comes from sand, the fluxes are typically carbonates, and the others are often found in their natural state.
The glass-forming system may thus be characterized as a purely physical admixture of ingredients, the physical state of each ingredient being dictated primarily by economics.
The various components are typically formulated from suitable admixtures.
In the case of a pure silicon glass, quartz may be used, but the temperature required for fusion thereof, which is on the order of 1800.degree. C., presents certain, truly vexing production problems.
Thus, in most cases, sand is employed as the starting material, to which mineral compounds are added in specific instances, such as feldspar, sodium carbonate and sulfate, limestone and dolomite, minium, sodium borate, etc.
The mixture is malaxated and gradually heated to a temperature conventionally on the order of 1500.degree. C., thus effecting melting or fusion, and a combination of the several ingredients, to form the glass. Because the molten mass contains many bubbles, an operation deemed fining is effected to eliminate the bubbles and the glass is then brought to a temperature imparting a particular viscosity thereto compatible with the actual forming operation. This is the heat-conditioning stage.
Numerous references relate to the fusion of vitrifiable admixtures. It has even been proposed to subject the materials to a preheating step prior to the actual melting [see French Patent No. 2,281,902], but to date little effort has been devoted to modifying the compositional nature, per se, of the glass-forming system.
It too is known to this art that the term "lead silicate" is intended to circumscribe many compounds of different chemical compositions and varying in their ratio SiO.sub.2 /PbO.
And even though these salts have long been studied, in reality only the lesser molar ratios, SiO.sub.2 /PbO, have seriously been investigated.
Thus, several basic lead silicates were especially noted by Mostowitsch in Metallurgie Halle, 4, pp. 649-53 (1907), as reported by Louis Cloutier, Annales de Chimie, 19, p. 41 (1933). But these silicates were obtained simply by melting varying admixtures of PbO and SiO.sub.2.
In the aforementioned reference, Cloutier describes a method of obtaining the silicates by precipitation from a solution of lead nitrate and the silicate, SiO.sub.3 Na.sub.2. But the only definite compound said to be obtained corresponded to the formula 3PbO.2SiO.sub.2, with the other silicates being considered as particular mixtures rather than as well-defined compounds. However, it has since been reported that it is possible to obtain a silicate close in composition to PbO.2SiO.sub.2, by precipitation from solutions of Na silicate and a 20% solution of Pb(NO.sub.3).sub.2 [Chemical Abstracts, 52 p. 145g (1958], but according to this process an excess of the lead salt induces the growth of Pb(OH)NO.sub.3 crystals.
Further, a process to prepare lead silicate from a solution of lead acetate and silicate too is quite old to the art; compare U.S. Pat. No. 1,895,910. Such a product is specified as a filler for an elastomer composition. Nonetheless, it will be appreciated that in the immediately aforesaid citations the silicates reported have low molar ratios of SiO.sub.2 to PbO, less than two.
In French Patent No. 1,187,368, it is mentioned that it is possible to prepare a pigment having 0.75 to 7.5 SiO.sub.2 molecules per PbO molecule, but such product is a pigment composed of lead phosphate and lead silicate containing PbO, P.sub.2 O.sub.5 and SiO.sub.2 in the form of a physico-chemical combination.
It is also known that sodium silicate, in particular, displays an increase in molecular weight in aqueous solution when the SiO.sub.2 /Na.sub.2 O ratio is increased, which makes it difficult to predict the effect of the initial solution on the final product.